Nuclear reactors consist of an array of fuel rods containing the radioactive nuclear fuel. The rods are commonly elongated slim metal tubes and are configured in groups, as in modular or unit fuel rod assemblies. After an extended period of reactor use, such irradiated or spent fuel assemblies must be lifted from the reactor proper and replaced while still retaining appreciable amounts of numerous fission products.
Such irradiated fuel assemblies have generally been stored in special liquid filled pools until they are to be reprocessed. Racks for such interim storage of bundles removed from nuclear reactors are known where the assembly receiving tubes stand on a bottom plate of the main storage pool or tank. They are usually braced laterally by structures connected to both the bottom plate and the tank walls, in anticipation of surviving seismic rare disturbances.
Recoverable Irradiated Fuel (RIF) assemblies are placed in storage and utilize pool water to fulfill four basic needs: (1) to provide nuclear isolation between RIF units; (2) to provide radioactive shielding to protect personnel; (3) to remove the heat buildup caused from radioactive decay of the fuel rods; and (4) to limit radioactive contamination to the storage pool and racks. The dependability of a liquid moderator, usually water, to meet these critical needs is contingent on the integrity of the storage pool. If the main reservoir level cannot maintain its functional safe level, e.g., because of a seismic event or piping system failure, then the RIF loaded rack will lose its liquid cover. Such a loss could lead to a critical configuration of the RIF units, leading to excessive heat buildup, possibly lethal radiation levels, and the consequent unregulated broadcast of radioactive contamination in the storage pool.
Because of these ominous risks of moderator loss, commercial facilities must then resort to emergency remedial actions to restore the required liquid cover over the irradiated fuel assemblies. Such steps include but are not necessarily limited to: (1) closing gates to isolate the affected storage; (2) moving single RIF units to an alternate storage pool; (3) moving a loaded storage rack to a functioning pool should the inherent rack design so permit; and (4) rapid refilling the entire reservoir to regain the liquid level essential to proper operation.
If such crash steps cannot be implemented prior to fuel rack exposure, site evacuation may be necessary due to rising radiation levels or the danger of reaching a critical fuel configuration. The dire consequences of even remote events like failure of the pool liquid level over nuclear fuel assemblies is a continuing concern in the nuclear industry, which has accordingly generated many corrective or protective approaches.
Among the prior art approaches to remedial design of nuclear fuel storage assemblies is the storage rack of U.S. Pat. No. 4,400,344 to Wachter, et al., disclosing a rack consisting of a checkerboard array of square storage cells or tubes. Alternate cells in each row include a neutron-absorbing poison material (usually in the tube wall), while the other cells are used for storage of the spent fuel assemblies. For temporary storage, the poison cells contain a moderator like water, and the entire rack is encased in concrete for shielding. The resultant 50 percent loss of rod storage space and the complex construction make evident the expense and the probable inability to withstand a major seismic mishap because of the vulnerability of the concrete casing to fracture.
Another approach to interim storage of rod assemblies is seen in U.S. Pat. No. 4,348,352 to M. Knecht, which discloses a rack installable in a water tank and which is designed to be earthquake proof, also receiving the fuel rod assemblies in close packing. The maintenance of the liquid level in the water tank is quite dependent upon the integrity of the reinforced concrete tank (reinforced with steel plate) for storing of the rack unit. Seismic safety is thus tied to massive steel sheet reinforced concrete storage pools, rather than to any improvement in rack configuration which would retain cooling water about irradiated rod assemblies despite unplanned liquid level loss about the rack.
U.S. Pat. No. 4,187,433 to Zezza discloses a high storage density nuclear fuel assembly in a pool with each fuel cell being vertically movable in the storage rack. Circulation openings at the top and bottom of the cells permit pool water circulation through them; of course, this is effective only so long as ambient water is available in the pool.
U.S. Pat. No. 4,318,492 to Peehs, et al., is directed to a fuel assembly storage capsule formed of a sleeve sealed at its bottom and having an upper opening closable by a cover that has a cross section matching that of the fuel assembly to be received therein. The sleeve and the cover rim define an annular space filled with air. No means for liquid circulation therethrough is taught.
U.S. Pat. No. 4,788,029 to Kerjean, et al., employs an apparatus for storing fuel assemblies in a pool where they are separated by a water gap. The walls of the cells are externally covered with a neutrophage material for irradiation control. There is no teaching relating to use of circulating liquid for waste heat removal and control.